For a method of diagnosing a mammary tumor, cirrhosis, a lesion of a blood vessel and others, a method (elastography) of diagnosing hardness inside a subject based upon an ultrasonic echo signal in place of palpation by a doctor can be given. In the diagnosis of hardness by elastography, an operator presses a probe on a surface of the subject and generates displacement in a tissue inside a measuring object in a living body and others (hereinafter called a conventional type method). Displacement in a direction of compression is estimated based upon echo signals before and after the compression of the tissue in the living body by pressure, distortion which is a space derivative of the displacement is calculated, and the distortion is imaged. This method has a problem that an imaged object is limited to internal organs that exist in locations in which pressure from a surface of the body is easy. For example, as a slide plane as an intervening layer exists between the surface of the body and the liver, it is difficult to press so that sufficient displacement is generated.
Then, technique for diagnosing hardness by applying radiation pressure to the inside of the subject using an ultrasonic focused beam and displacing a target tissue, inhibiting an effect of the intervening layer can be given. For example, there is acoustic radiation force impulse (ARFI) imaging disclosed in a patent literature 1. In this technique, the displacement of a tissue generated in a direction in which a focused beam advances is imaged and a modulus of elasticity such as a modulus of elasticity in shear and Young's modulus is calculated based upon the estimate of the propagation velocity of a shear wave generated in a direction perpendicular to the direction in which the focused beam advances according to the displacement of the tissue at a focus. When this technique is used, diagnosis in which dependence upon manual technique is reduced is expected because the tissue is displaced by ultrasound in addition to the effect of reducing the effect of the intervening layer such as the slide plane.
When heterogeneity related to sonic velocity exists in the tissue in a measuring range, the measured modulus of elasticity has a value including the propagation velocity of plural shear waves. For a cause of the heterogeneity of sonic velocity, tissue structure, frequency dispersion, an amplitude, particle velocity and others can be given. For example, as for frequency dispersion, a nonpatent literature 1 and a nonpatent literature 2 can be given.